Large fossil-fuel consumers, such as thermal power plants and ironworks, burn fossil fuels in boilers to generate combustion discharge gases. Those gases as well as natural gases and coal gasification gases (gasification gases) contain acidic gas components, such as, carbon dioxide (CO2), SOx, NOx and H2S. In order not to emit those acidic gas components in the discharge gases into the atmosphere, methods for recovering the acidic gas components have been vigorously studied and developed. In those methods, discharge gases containing the acidic gas components are introduced into an absorption tower and brought into gas-liquid contact with an absorbing liquid containing an amino group-containing compound (amine-based compound) so that the absorbing liquid may absorb the acidic gas components to remove them from the treated gases.
For example, there is a known carbon dioxide recovery apparatus comprising: an absorption tower in which a discharge gas is brought into contact with an absorbing liquid containing an amino group-containing compound so that the absorbing liquid can absorb the acidic gas components such as carbon dioxide; and a regeneration tower in which the absorbing liquid loaded with the absorbed acidic gas components is heated to release the acidic gas components and thereby to be regenerated. The regenerated absorbing liquid is then supplied again to the absorption tower and reused there. In the apparatus, the absorbing liquid is thus circularly used in the system including the absorption and regeneration towers.
However, the above carbon dioxide recovery apparatus has a problem in that the discharge gas from which carbon dioxide is absorbed and removed by the amine-based absorbing liquid in the absorption tower, namely, the CO2-removed discharge gas (decarbonated combustion discharge gas) is finally emitted together with the amine from the absorption tower into the atmosphere. Accordingly, since thermal power plants and the like exhaust a large amount of decarbonated combustion discharge gas, they may also release a large amount of amine at the same time. It is hence necessary to effectively reduce the amine emitted together with the decarbonated discharge gas into the atmosphere. In view of that, there is, for example, a known cleaning apparatus in which the decarbonated discharge gas is brought into contact with a cleaning liquid so as to recover the amine accompanying the discharge gas.
Further, when carbon dioxide in the discharge gas is absorbed in the recovery apparatus, not only SOx and NOx but also other inorganic acids, such as, carbonyl sulfide, hydrogen cyanate, thiocyanic acid and thiosulfuric acid, may react with the amino group-containing compound contained in the absorbing liquid to form degradation products referred to as “heat stable amine salts: HSAS”. Further, when the absorbing liquid is heated to be regenerated, the amino group-containing compound may be decomposed thermally or by reaction with oxygen in the discharge gas and, as a result of that, heat stable amine salts are also formed.
The heat stable amine salts are not thermally decomposed in the heating procedure for regenerating the absorbing liquid in the regeneration tower, and hence are not removed from but accumulated in the absorbing liquid. The heat stable amine salts thus accumulated not only impair the absorbing liquid to lower the efficiency of acidic gas absorption but also cause corrosion of the apparatus. It is hence desired to remove the heat stable amine salts from the absorbing liquid.
As a method for removing the heat stable amine salts from the absorbing liquid, it is known to adopt an electrodialyser of three-compartment structure, for example. The electrodialyser comprises a bipolar membrane in combination with plural ion-exchange membranes, and has three compartments, namely, an amine-purification compartment, an amine-recovery compartment and an acid-recovery compartment. Those compartments are provided between the electrodes facing each other, and aligned in the above order from the cathode side to the anode side. In the electrodialyser, the absorbing liquid undergoes electrodialysis so that the heat stable amine salts migrate from the absorbing liquid to a concentrate to be removed.